The present invention relates to a surgical implant with a metallic base. More specifically, this invention relates to a surgical implant coated with a bioactive ceramic layer which exhibits good adhesion to the metallic implant base.
Prior art surgical implants coated with a bioactive ceramic layer have been used in order to enhance their affinity to bone. Such prior art surgical implants usually have a metallic implant base fabricated from stainless steel, Co--Cr alloy, Ti or a Ti alloy. The metallic implant base is then coated with a ceramic layer consisting essentially of hydroxyapatite or calcium phosphate. Ceramic layers which have added material such as trace amounts of alumina, zirconia or titanic are generally classified as bioactive ceramic materials.
Hydroxyapatite is the most commonly used bioactive ceramic material. Its use is predicated upon its excellent affinity for bone. This property is due to the fact, that hydroxyapatite is a major component of the bones and teeth of living animals.
Hydroxyapatite can be prepared in numerous ways. The most common process for preparing hydroxyapatite is called the wet process. In this process, Hydroxyapatite is produced when a mixture containing CaHPO.sub.4 and ammonium hydrogen phosphate is allowed to react. In yet another process, called the dry process, Ca.sub.3 (PO.sub.4).sub.2 is allowed to react with CaO in a steam atmosphere at a temperature not lower than 900.degree. C., thereby producing Hydroxyapatite.
Hydroxyapatite can also be prepared by means of a biological process. Japanese laid-open patent publication No. 58-39533 discloses such a process. In a biological extraction process organic matter is extracted from the bones of animals such as an ox or a horse by means of a chemical agent.
Alternatively, the organic matter can be extracted by burning. The powdery hydroxyapatite thus obtained is then welded to the surface of a metallic implant base by spraying or sintering. The resulting metallic implant base has a porous hydroxyapatite layer on its surface.
When used as a surgical implant, this type of implant allows the bone structure to migrate into and proliferate in the pores of the porous hydroxyapatite layer, and anchor itself to the implant. This feature in turn, allows the implant to firmly integrate the underlying bone.
However, use of the aforementioned surgical implant has numerous disadvantages. Chief among them, is the inability of the hydroxyapatite to effectively bind to the metallic implant base. This is due to the porosity of the hydroxyapatite layer. As a result, the surgical implant is not strong and effective in terms of imparting strength to the underling skeletal framework.
Additionally, metal ions from the metallic implant base tend to invade the surrounding bone cells. It is worth noting, that this feature has been linked to bone cancer.
Numerous prior art surgical implants have attempted to overcome the aforementioned deficiencies by providing for an inner and an outer ceramic coated layer.
For instance, Japanese laid-open Patent Publication No. 58-50737 discloses a surgical implant wherein the improvement lies in spraying an inner ceramic layer onto the surface of the metallic implant base. The ceramic layer is made from a ceramic material selected from a group consisting of Al.sub.2 O.sub.3, TiO.sub.2, ZrO.sub.2 and SiO.sub.2, all of which are non-porous. This inner ceramic layer is supplemented with a porous layer containing hydroxyapatite.
Japanese laid-Open Patent Publication No. 63-160666 also attempts to provide an improved surgical implant. It discloses a surgical implant spray coated with an inner ceramic powder layer, which is further coated with a biologically incompatible metal layer. The inner ceramic powder layer and the biologically incompatible metal layer is supplemented with a outer porous hydroxyapatite layer.
The attempted improvement in the aforementioned surgical implant lies in forming a coat of a biologically incompatible metal layer on a ceramic powder layer, which is then spray coated onto a surface of the metallic base implant. The biological incompatibility of the metal layer is thought to prevent the invasion of the underlying bone cells by the metal ions.
However, the abovementioned surgical implants have numerous disadvantages. Chief among them is the cost of manufacturing the implants. The added cost is attributed to the necessity for coating a powdery ceramic layer with a biologically incompatible metal plating prior to forming an outer porous hydroxyapatite layer on the inner ceramic powder layer.
Additionally, the outer porous hydroxyapatite layer tends to separate when subjected to impact. This is due to its inability to effectively bond to the ceramic powder layer laced with a coat of a biologically incompatible metal.